Monitoring cerebral oxygenation offers great promise in the management of acute, life-threatening neurologic illnesses, including severe traumatic brain injury. To date, the two primary methods used to monitor brain oxygenation are invasive- one requires percutaneous insertion of a catheter into the jugular bulb to continuously measure cerebral venous oxygenation and the other requires insertion of a probe through the skull into the brain parenchyma to measure tissue PO2. The first technique requires frequent recalibration. Second one cannot indicate inadequate tissue oxygenation in remote sites. Near- infrared spectroscopy, a third, noninvasive method of monitoring cerebral blood oxygenation, is promising, but has yet to be satisfactorily calibrated to provide quantitative measurement. Therefore, at present there is no system for accurate, non- invasive, and continuous monitoring of cerebral blood oxygenation. We propose a novel technique that will utilize well-established differences in optical absorption coefficients of oxy- and deoxyhemoglobin in the near-infrared spectral range to accurately monitor blood oxygenation. The objective of this project is to test a novel optoacoustic technique to measure the absolute value of cerebral venous oxygenation directly from the superior sagittal sinus (SSS), a structure that can be localized due to the high resolution of the optoacoustic technique. The optoacoustic technique is based on generation of ultrasonic waves by laser pulses and detection of these waves by a sensitive acoustic transducer. Our preliminary studies demonstrate that (1) the amplitude and temporal profile of the ultrasonic waves are linearly dependent on blood oxygenation; (2) the use of specially designed transducers allows sensitive detection despite optical and acoustic attenuation by thick bone; and (3) this technique will measure blood oxygenation with high accuracy. The specific aims of the project are: (1) to evaluate optoacoustic monitoring of blood oxygenation in backward mode in vitro in phantoms; and (2) to evaluate optoacoustic monitoring of blood oxygenation in backward mode in vivo in sheep. Performance will be evaluated in normal sheep and in experimental models in sheep that replicate conditions typical of severe venous desaturation.